1. Field of the Invention
The present invention relates to a photosensitive body for electrophotography and more specifically to an organic photosensitive body for electrophotography which includes a photosensitive layer having a novel structure and which is used in a positively charged state.
2. DESCRIPTION OF PRIOR ART
A photosensitive body for electrophotography (hereinafter may be referred to simply as photosensitive bodys) for use in electrophotographic apparatus that was begun with the invention of Carlson, there have been widely employed photosensitive bodys comprising inorganic photoconductive materials such as selenium, selenium alloys, zinc oxide, and cadmium sulfide. Recently, however, photosensitive bodys comprising organic photoconductive materials have been developed energetically because of their nontoxic nature, film-forming properties, light weights, low prices, and so on. Among them are so-called double-layered laminate type organic photosensitive bodys having a photosensitive layer divided into a charge generation layer for receiving light to generate charge carriers therein, and a charge transport layer for transporting the charge carriers which are generated in the charge generation layer. The photosensitive bodys of this type have many advantages such that their sensitivity can be enhanced markedly by combining the respective layers formed of materials optimal for their functions, and that their spectral sensitivity can be increased in response to the wavelength of exposing light. Thus, they have become the mainstream of development, and their practical use is under way.
Many of the double-layered laminate type organic photosensitive bodys that have now found practical use comprise a charge generation layer and a charge transport layer in this order arranged on a conductive substrate. This layered structure is preferred, because it is advantageous in that the charge generation layer in the form of a thin film with a thickness of 1 .mu.m or less can be protected with the charge transport layer with a film thickness of several tens of micrometers. At the time of image formation, the photosensitive body is usually charged negative on its surface. This is because the charge transfer substance of the charge transport layer now in practical use has hole mobility.
For the purpose of image formation, the surface of the photosensitive body is charged normally by a corona discharge. A corona discharger such as a corotton discharger or a scorotron discharger gives off ozone. Compared with a positively charged surface of the photosensitive body, its negatively charged surface causes an enormous amount of ozone, posing the problem of ozone-associated considerable deterioration of the photosensitive body and the environment. Moreover, an image forming process based on negative charging requires a toner of a positive polarity for developing the image. It is difficult that such a toner of a positive polarity is produced. Even if such toner is produced, it will not have uniform properties.
To resolve the above-mentioned problems, various proposals have been made for organic photosensitive bodys which can be used in a positively charged state. For example, there has been proposed a positively charged photosensitive body with a photosensitive layered structure reverse to that of a conventional negatively charged photosensitive body, i.e., a photosensitive body having a photosensitive layer comprising a charge transport layer having hole mobility, and a charge generation layer arranged on the charge transport layer, the charge generation transport for generating charge carriers when receiving light. However, such a photosensitive body has the charge generation layer exposed on the surface. Thus, it is apt to be affected by ultraviolet radiation during illumination, ozone generated by a corona discharge during charging, and humidity in the surrounding environment. It is also vulnerable to external actions such as mechanical frictions during development, transfer or cleaning. Thus, the electrical characteristics and image characteristics of the photosensitive body degrade noticeably, eventually leading to its poor durability.
To eliminate these drawbacks, it has been proposed to provide on the charge generation layer a protective layer comprising an insulating or conductive transparent resin layer. For instance, Japanese Patent Application Laying-open Nos. 211561-4/1991 propose providing a protective layer containing picric acid, phthalic anhydride, hydrophobic silica and nitrobenzoic acid, respectively, added to a curable silicone resin to impart conductivity. Likewise, Japanese Patent Application Laying-open No. 157664/1991 proposes providing a thin film of diamond as a protective layer by the CVD (Chemical Vapor Deposition) method. These methods forming a protective layer essentially induce a decrease in the sensitivity of the photosensitive body, and pose the problems that too thick a protective layer results in a sharp decrease in sensitivity, while too thin a protective layer exhibits poor function.
Also, a photosensitive body has been proposed as a positively charged photosensitive body, including a charge generation layer and a charge transport layer containing 2,4,7-trinitro-9-fluorenone with high electron mobility which is arranged on the charge generation layer. However, the substance is carcinogenic, and cannot be used actually from the viewpoint of public health. Electron-mobile substances free from carcinogenicity have also been eagerly developed. For example, Japanese Patent Application Laying-open No. 335639/1992 shows an electron-mobile polymer, Japanese Patent Application Laying-open No. 338761/1992 indicates an example of using a cyclic sulfone as an electron-mobile substance, and Japanese Patent Application Laying-open No. 331958/1992 reveals an example of using a cyanoimine as an electron-mobile substance. Japanese Patent Application Laying-open Nos. 61218/1993 and 61219/1993 show examples of using aromatic or vinyl compounds having potent electron attractive groups as electron-mobile substances. These substances, however, have not a little carcinogenicity, and their synthesis is also difficult. Thus, the production of positively charged photosensitive bodys on a commercially feasible scale leaves much to be desired.
Japanese Patent Application Laying-open Nos. 102360/1991, 58054/1991 and 122948/1992 propose single-layered photosensitive bodys comprising pyrylium salts as charge generation substances, and photosensitive layers containing eutectoid complexes of these salts with binder resins. Such photosensitive bodys have the drawback of a high memory effect.